Viruses

Viruses no cellular structure, no cytoplasm, organelles or cell membranes no metabolism on their own, cannot grow or respire Therefore are not classified as living things

Viruses continued Viruses can reproduce, a basic characteristic of life They can take over control of a host cell Can direct cell to make new virus particles Mobile genes that attack cells

Parts of a virus Most viruses consist of 4 parts Core - located in the center of the virus and contains the virus’ DNA or RNA wrapped together with some proteins 2) Capsid - made of protein and forms a shell around the core, protects DNA from enzymes of host cell 3) Matrix - forms a layer between the capsid and the envelope 4) Envelope - consists of lipids stolen from the cell membrane of the host cell that the virus is growing in Note: some viruses do not have an envelope and others do not have a capsid

Type of disease caused (poliovirus) Classifying Viruses Often viruses are named for: Type of disease caused (poliovirus) Those who discovered them (Epstein-barr virus) Site of replication (rhinovirus or tobacco mosaic virus) Where they were isolated (sendai-virus) Classification is now based on type of nucleic acid, shape, size etc.

Shape and Size. --> determined by the type and Shape and Size --> determined by the type and arrangement of proteins in the capsid Cylindrical Polyhedral Spherical

Site of Replication Specific organism and cell Wide variety of organisms e.g. Tobacco mosaic virus

Type of disease they cause 21 different groups of viruses infecting humans differ in set of genes (genome) and method of replication e.g. influenza virus

Type of nucleic acid DNA viruses RNA viruses Less virulent follow the lytic cyle Eg. Warts, shingles, chicken pox RNA viruses Virulent Follow lysogenic cycle AIDS, rubella, rabies, measles

How big are viruses? Video

Viral Replication It depends on the metabolism of a prokaryotic or eukaryotic cell to replicate its DNA or RNA and to make protein coats foe each newly formed virus particle Attaches to specific receptors on the plasma membrane of the host cell and can only enter particular cells with specific receptor sites Enter host in one of two ways a) the virus injects its nucleic acid into host cell (lytic cycle) b) membrane of host engulfs virus creating a vacuole inside host Host metabolism replicates the viral DNA or RNA and protein coat and new virus particles are assembled

New particles are released from the host cell in one of two ways: The host cell breaks open (Lysis) and releases the new viruses which then infect neighbouring cells; Host cell is destroyed in process The host cell releases new viruses without being destroyed

Bacteriophage Virus that infects a bacteria Video Same video different site

T4 Bacteriophage Specific to E.coli Head  capsid containing DNA Sheath  support and pump to move DNA Base plate  cut or bite a hole in cell Tail fibres  find site for attachment Core  pipeline to conduct DNA from head to cytoplasm of Collar attach tail to head

Viral Replication: Lytic and Lysogenic Cycles Video

Viroids Very small infectious pieces of RNA responsible for some serious plant diseases Differ from viruses as RNA does not code for proteins May interfere with the normal formation and functioning of RNA in host cell Eg. Viroid outbreak killed more than 10 million coconut plants in Philippines

Prions Abnormally shaped infectious protein responsible for some brain diseases in mammals, including humans When tissues are eaten by another animal, prions enter blood stream and go to its brain Prions interact with normal proteins causing them to become abnormal and infectious E.g. Bovine spongiform encephalopathy (BSE) (Mad Cow Disease) Humans - Creutzfeldt-Jakob Disease (CJD)